a) Field of the Invention
The present invention relates to apparatus and method for detecting (measuring) a displacement.
b) Description of the Related Art
Each of U.S. Pat. Nos. 4,676,645 and 5,432,443 exemplifies an optical or magnetic instrument for measuring a displacement of an object to be measured.
In a previously proposed displacement measuring instrument, two equilibrium modulated signals representing a result of displacement measurement are converted into a phase modulated signal, a period of the phase modulated signal being compared with that of a reference signal so as to detect the displacement of the measured object.
That is to say, a difference in period of both phase modulated signal and reference signal represents the displacement of the measured object.
In order to improve an accuracy of detection of the displacement, an interpolation processing is executed.
Previously proposed methods of outputting an interpolation data include: an outputting method of a clock pulse and a direction discrimination signal and an outputting method using a two-bit Gray code (A/B phase).
In the latter method, the interpolated displacement is achieved for each period of the phase modulated signal. When the displacement is equal to one or less, the A/B phase is varied at a period of the phase modulated signal multiplied by an integer. When the displacement is equal to two or more, the A/B phase is varied at a period of a clock inputted into a pulse train converter.
The clock inputted into the pulse train converter is set such that a frequency of the clock is set to that of the pulse modulated signal multiplied by several tens, namely, a period of the clock is set to that of the pulse modulated signal multiplied by several tenths.
However, if a movement speed of a scale used to measure the displacement of the object to be measured exceeds a certain speed, a period of the clock pulse is abruptly short, and a phase difference time of the A/B phase signal is abruptly short, and a higher frequency clock is furthermore needed.
In the case where the clock pulse and direction discrimination signal are outputted and the movement speed exceeds a certain speed and the movement speed of the scale exceeds the certain speed, the period of the clock pulse needs to be made shorter so that a processing capability more than necessary is required for a receiving side of the clock pulse and direction discrimination signal.
On the other hand, in the case where the A/B phase signal is outputted and the movement speed of the scale exceeds the certain speed so that the phase difference time of the A/B phase signal becomes shorter, the processing capability more than necessary is required for the receiving side of the clock pulse and direction discrimination signal.
Consequently, a cost of the whole system of the displacement measuring instrument is increased.